Facile construction of functionalized periodic mesoporous organosilica for Ir-catalyzed enantioselective reduction of α-cyanoacetophenones and α-nitroacetophenones

Article abstract of DOI:10.1039/c4cc04169k

A facile method to construct chiral organoiridium-functionalized periodic mesoporous organosilica is developed. The heterogeneous catalyst displays excellent catalytic efficiency in the enantioselective reduction of α-cyanoacetophenones and α-nitroacetoph

Full text of DOI:10.1039/c4cc04169k

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Facile construction of functionalized periodic
mesoporous organosilica for Ir-catalyzed
enantioselective reduction of a-cyanoacetophenones
and a-nitroacetophenones†
Cite this: Chem. Commun., 2014,
50, 10891
Received 31st May 2014,
Accepted 24th July 2014
Chen Chen, Lingyu Kong, Tanyu Cheng, Ronghua Jin and Guohua Liu*
DOI: 10.1039/c4cc04169k
www.rsc.org/chemcomm
A facile method to construct chiral organoiridium-functionalized anchored onto a metal-functionalized PMO. No chiral ligand-
periodic mesoporous organosilica is developed. The heterogeneous derived siloxane is necessary in this process. Taking advantage
catalyst displays excellent catalytic efficiency in the enantioselective of the post-coordination method and the hydrophobic feature of
reduction of a-cyanoacetophenones and a-nitroacetophenones in the PMO network, we conveniently construct a hydrophobic, chiral
aqueous medium because of the hydrophobic nature and uniformly organoiridium-functionalized heterogeneous catalyst through the
distributed active iridium species. The catalyst could be conveniently direct post-coordination of commercial (R,R)-pentafluorophenyl-
recovered and reused eight times without loss of its catalytic activity. sulfonyl-1,2-diphenylethylenediamine (PFPSDPEN) onto an organo-
iridium-functionalized PMO. As expected, the high hydrophobicity
Great achievements have been made in immobilization of chiral and uniformly distributed active iridium species greatly promote
organometallic complexes using periodic mesoporous organo- the enantioselective reduction of a-cyanoacetophenones and
silicas (PMOs) as supports for constructing heterogeneous chiral a-nitroacetophenones in aqueous medium. In addition, the
catalysts.1 Such achievements are due to the similarity of physical heterogeneous catalyst could be recovered and reused at least
properties of PMOs to those of inorganosilica mesoporous mate- eight times without loss of its catalytic activity.
rials, such as large specific surface area and pore volume, tunable
Ethenylene-bridged PMO functionalized with chiral
pore dimension, well-defined pore arrangement, as well as high Cp*IrPFPSDPEN, abbreviated as Cp*IrPFPSDPEN–PMO (3),
thermal and mechanical stabilities.2 Furthermore, the potential (Cp*IrPFPSDPEN:6 Cp* = pentamethyl cyclopentadiene, PFPSDPEN =
hydrophobicity of PMOs due to the intrinsic organosilicate inner (R,R)-pentafluorophenylsulfonyl-1,2-diphenylethylenediamine), was
surface significantly promotes organic transformation in biphasic prepared as outlined in Scheme 1. First, ethenylene-bridged PMO (1)
reaction media.3 In general, immobilization of chiral organome- was obtained through the co-condensation of 1,2-bis(triethoxysilyl)-
tallic complexes within the PMO network can be achieved either by ethylene according to a reported method.5b Epoxidation of double
a post-grafting strategy or by a co-condensation strategy,1a,b which bonds (–CQC–) followed by conversion of the resulting epoxide with
have led to many highly efficient heterogeneous chiral catalysts bisulfite ions then afforded 2 in the form of a white powder.7 Finally,
in various asymmetric reactions. From the synthetic methods, continuous ion exchanges with Ag2O8 andthenwith(Cp*IrCl2)2 (ref. 9)
assembly of a PMO-based heterogeneous chiral catalyst often followed by direct complexation of PFPSDPEN led to the crude
requires chiral ligand-derived siloxanes, regardless of the strategy heterogeneous catalyst 3. This was subjected to Soxhlet extraction to
employed.4 However, preparation of a pure chiral siloxane derived clear its nanochannels and to obtain its pure form as a light-gray
from a chiral ligand is very difficult because of the tedium of its powder (see ESI† in the experimental and in Fig. S1–S5).
purification by silica gel column chromatography. Thus, explora-
tion of an alternative means to directly anchor a chiral ligand for within the PMO network could be proven by solid-state 13C cross-
constructing heterogeneous chiral catalysts is highly desirable. polarization (CP)/magic angle spinning (MAS) NMR spectroscopy. As
Incorporation of well-defined, single-site, active iridium centers
In our effort to assemble heterogeneous catalysts through shown in Fig. 1, catalyst 3 produced strong, characteristic carbon
various chiral siloxanes,5 we utilize a post-coordination method signals of SiCHQCHSi groups at B145 ppm, corresponding to the
À
À
in the present contribution. That is, a chiral ligand was directly main ethenylene-bridged organosilica embedded within the PMO
network. Additional weak carbon signals observed between B43 and
B51 ppm are attributed to the SiCHOH and SiCHSO3H groups,
À
À
Laboratory of Resource Chemistry of Ministry of Education,
indicating that sulfonation occurred.7 Both observations reveal that
only a fraction of –CQC– double bonds underwent sulfonation
during synthesis (corresponding to weak carbon signals), while those
Shanghai Normal University, Shanghai 200241, China. E-mail: ghliu@shnu.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures
and analytical data of chiral alcohols. See DOI: 10.1039/c4cc04169k
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parent counterpart, weak T1(R(HO)2SiOSi), Q3((HO)Si(OSi)3), and
Q4(Si(OSi)4) signals could be observed in this case.7
In addition, catalyst 3 produced a IV-type nitrogen adsorption–
desorption isotherm with a H2 hysteresis loop (see ESI† in Fig. S3)
while its small-angle X-ray diffraction (XRD) pattern (see ESI† in Fig. S4)
exhibited a well-resolved peak at 2y = 0.81–1.01, indicating its ordered
dimensional-hexagonal (P6mm) mesoporous channels.5b,7 Transmis-
sion electron microscopy (TEM) images further confirmed the ordered
dimensional-hexagonal pore arrangements (Fig. 2a and b). It is worth
mentioning that the TEM image with a chemical mapping revealed
that the iridium active centers were uniformly distributed within the
PMO network (Fig. 2c), demonstrating the formation of site-isolated
active species proved by its 13C CP/MAS NMR spectrum.
Table 1 summarizes the general applicability of catalyst 3 in the
enantioselective reduction of a-cyanoacetophenones and a-nitroaceto-
phenones in aqueous medium, in which the asymmetric reaction with
the hydrogen source HCO2H and 0.25 mol% PMO as a catalyst was
carried out according to a reported method.6a In general, high conver-
sions, no side products, and high enantioselectivities were obtained for
all tested substrates. Taking enantioselective reduction of benzoylace-
tonitrile as a example, it was found that catalyst 3 produced (R)-3-
hydroxy-3-phenylpropanenitrile with 499% conversion and 96% ee
value. Such an ee value is slightly higher than that of Cp*IrPFPSDPEN
(entry 1 vs. entry 1 in brackets, Table 1), even higher than that obtained
with the mixed SO3H–PMO (2) and its homogeneous Cp*IrPFPSDPEN
as a catalyst (entry 2), indicating that the coordination environment of
catalyst 3 remained in its original homogeneous situation that could
be confirmed by X-ray photoelectron spectroscopy (XPS) investi-
gation (see ESI† in Fig. S5). In addition, the asymmetric reaction
could be run at much higher substrate-to-catalyst mole ratios
without apparently affecting its ee value, as exemplified by the
enantioselectivereduction ofbenzoylacetonitrile at a substrate-to-
catalyst mole ratio of 600 (entry 3, Table 1).
Scheme 1 Preparation of catalyst 3.
embedded within the pore walls were inaccessible for sulfonation
(corresponding to strong carbon signals). Signals for carbon atoms
of NCHPh groups (between B69 and B73 ppm) and for carbon
À
atoms of the aromatic ring (at B129 and B138 ppm) of the chiral
PFPSDPEN moiety could be observed clearly (marked in the spectra).
Peaks at B87 and B11 ppm are ascribed to the carbon atoms of the
Cp ring and to the carbon atoms of the CH3 groups attached to the
À
Cp ring, respectively. These peaks are absent in the spectrum of 2,7
suggesting the formation of the Cp*IrPFPSDPEN complex. Chemical
shifts of 3 are similar to those of its homogeneous counterpart,
Cp*IrPFPSDPEN,6a demonstrating that 3 had the same well-defined
single-site active species of Cp*IrPFPSDPEN. Its solid-state 29Si MAS
NMR spectrum further demonstrates its organosilicate network and
the composition of active Cp*IrPFPSDPEN (see ESI† in Fig. S2). Here,
two strong T signals, corresponding to T2(R-Si(OSi)2(OH)) and T3(R-
Si(OSi)3) (R = Cp*IrPFPSDPEN-functionalized alkyl-linked groups or
ethenylene-bridged groups), reveal that R-Si(OSi)2(OH) and R-Si(OSi)3
comprise the main organosilicate network.10 Similar to those of the
It is noteworthy that the electronic properties of substituents at
the Ar moiety of acetophenones did not affect enantioselectivities,
Fig. 2 TEM images of catalyst 3 viewed along the [100] (a) and [001] (b)
directions. (c) TEM image with a chemical mapping of 3, showing the
distribution of Si (white) and Ir (green).
Fig. 1 13C CP/MAS NMR spectrum of catalyst 3.
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Table 1 Enantioselective reduction of a-cyano and a-nitroacetophenonesa
the amount of Ir in the eighth recycle is 30.09 mg per gram of
catalyst and only 6.7% of Ir was lost.
In conclusion, by taking advantage of a direct post-coordination
method, we conveniently constructed one ethenylene-bridged, hydro-
phobic, chiral organoiridium-functionalized heterogeneous catalyst.
This catalyst displays excellent catalytic activity and high enantios-
electivity in the enantioselective reduction of a-cyanoacetophenones
and a-nitroacetophenones in aqueous medium. As presented in this
study, the excellent catalytic activity and high enantioselectivity are
attributed to the high hydrophobicity and uniformly distributed
single-site iridium active species within the PMO material, which
significantly promote organic transformation in aqueous medium. In
addition, the heterogeneous catalyst could be recovered and reused at
least eight times without loss of its catalytic activity. This strategy here
offers a facile means to construct a hydrophobic chiral organometal-
functionalized PMO with high catalytic performance.
Entry
1
2
3
4
5
6
7
Ar
X
Time
12
Conv.b (%)
99
eeb (%)
96(94)c
Ph
Ph
Ph
p-FPh
m-ClPh
p-BrPh
p-MePh
m-MeOPh
2-Furyl
2-Thiophenyl
Ph
CN
CN
CN
CN
CN
CN
CN
CN
CN
CN
NO2
NO2
NO2
NO2
NO2
14
24
12
12
12
12
12
12
15
10
10
10
10
15
99
92
99
99
99
99
99
99
99
99
99
99
99
99
92d
95e
93
93
93
97
94
96
98
97
96
93
96
96
8
9
10
11
12
13
14
15
p-FPh
p-ClPh
p-MePh
p-MeOPh
We are grateful to Shanghai Sciences and Technologies
Development Fund (12nm0500500 and 13ZR1458700), CSIRT
(IRT1269) and Shanghai Municipal Education Commission
(12ZZ135, 14YZ074) for financial support.
a
Reaction conditions: catalyst 3 (20.0 mg, 2.0 mmol of Ir, based on the
ICP analysis), a-cyanoacetophenones (a-nitroacetophenones) (0.80 mmol)
and the aqueous solution of formic acid (5.0 equiv. 1.0 M formate
solution, 0.2 M overall concentration, for X = CN, pH = 3.5; for X =
b
NO2, pH = 2.0), at room temperature (25 1C) for 10–24 h. Determined by
chiral HPLC analysis (see ESI in Fig. S11 and S13). Data in the bracket
c
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d
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that is, various electron-withdrawing and electron-donating sub-
stituents of the Ar moiety led to the same efficiency (entries 4–10).
The asymmetric reaction was also suitable for the enantioselective
reduction of a-nitroacetophenones. Similarly, high conversions,
no side products, and high enantioselectivities could also be
obtained with several a-nitroacetophenones (entries 11–15).
It is worth mentioning that the asymmetric reaction catalyzed by
3 has the reaction rate higher than that attained with its homo-
geneous counterpart, Cp*IrPFPSDPEN.6a For example, we found
that the enantioselective reduction of benzoylacetonitrile catalyzed
by 3 could reach completion within 12 h, whereas the reaction
catalyzed by Cp*IrPFPSDPEN required 24 h. Notably, the greatly
enhanced reaction rate attained with 3 is due to the uniformly
distributed single-site iridium active species and the highly hydro-
phobic PMO network (see ESI† in Fig. S6–S9). To confirm this
conclusion, the kinetic profile of the enantioselective reduction of
benzoylacetonitrile catalyzed by 3 and by Cp*IrPFPSDPEN were
investigated. The results show that the initial activity of 3 was
higher than that of Cp*IrPFPSDPEN; the initial TOF values were
94.2 and 39.0 mol (molÀ1 hÀ1), respectively (see ESI† in Fig. S10).
Important considerations in the design of any heterogeneous
catalyst are ease of separation by simple filtration and retention of
catalytic activity and enantioselectivity of the recovered catalyst
after multiple cycles. We found that 3 could be recovered easily
and reused in eight consecutive reactions for enantioselective
reduction of benzoylacetonitrile. In the eighth recycle, catalyst 3
still afforded the desired products with 98.2% conversion and
95.2% ee value (see ESI† in Table S1 and Fig. S12). Apparently, the
high recyclability should be due to the low leaching of Ir, in which
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